Monitoring device for emergency back-up grid operation

Abstract

In order to determine an AC insulation resistance (Rf) between an AC terminal (AC1, AC2) of the inverter (2) and a ground potential (GND), an insulation test (I) is performed in a network emergency operation (NE), wherein the AC terminal (AC1, AC2) is connected to the AC energy store (4) in the network emergency operation (NE) in order to transfer energy from the DC energy source (2) to the AC energy store (4), wherein an AC fault current (i fAC ) flowing between the AC terminal (AC1, AC2) and the ground potential (GND) is determined and the AC insulation resistance (R isoAC ) is calculated from the AC fault current (i fAC ).

Description

Technical Field

[0001] This invention relates to a method for determining the AC insulation resistance between the AC terminals of an inverter and the ground potential, wherein a DC voltage is applied to the DC terminals of the inverter and an AC voltage is applied between the AC terminals of the inverter, the DC terminals are connected to a DC energy source, and during emergency backup grid operation, the AC terminals are connected to an AC energy reservoir to transfer energy from the DC energy source to the AC energy reservoir. Furthermore, this invention relates to a monitoring device for determining the AC insulation resistance between the AC terminals of an inverter and the ground potential, wherein the DC terminals of the inverter can be connected to a DC energy source, and during emergency backup grid operation, the AC terminals of the inverter can be connected to an AC energy reservoir. This invention also relates to an inverter with a monitoring unit. Background Technology

[0002] An inverter, also known as a converter, is a DC / AC converter that converts an input DC voltage (DC voltage) applied between the DC terminals to the input side into an output AC voltage (AC voltage) applied between the AC terminals. The inverter's DC terminals are connected to a DC voltage energy source (DC energy source), such as a photovoltaic system operating as a generator. The DC energy source provides the DC voltage and powers the inverter. In grid operation, the inverter's AC terminals are connected to the power grid to feed the energy provided by the DC energy source into the grid. The AC voltage is advantageously synchronized with the grid voltage. If the inverter is designed, for example, as a three-phase inverter, then one AC terminal is provided for each phase, and each AC terminal is connected to a specific phase of the power grid, where the neutral wire can also be used as an AC terminal.

[0003] However, in emergency standby grid operation, the inverter's AC terminals are disconnected from the power grid and instead connected to an AC voltage energy bank (AC energy bank). Energy supplied by the DC power source is then output to the AC energy bank by the inverter. The disconnection of the AC terminals from the power grid is entirely polarized and can be done internally within the inverter or externally. It is also possible for multiphase inverters to operate in single-phase mode during emergency standby grid operation. To achieve single-phase operation, for example, only two of the three phase branches can be operated, where a single-phase output AC voltage is generated between two of the four AC terminals (three phases and one neutral).

[0004] Power grid protection devices are installed in the power supply network, which may include fault current protection switches, fuses, etc. Because the power grid protection devices are located on the grid side, they are only effective during grid operation, but ineffective during emergency backup grid operation, as all AC connections are separated from the power supply network.

[0005] Therefore, inverters often require protective devices to protect and / or monitor the inverter and, in particular, its AC terminals during emergency standby grid operation, when grid protection devices are not available for use in grid operation. Only some suitable considerations are derived from the following existing technologies.

[0006] Therefore, DE102014109513A1 describes a method and apparatus for monitoring power lines, but these power lines are components of a vehicle, not an inverter. Consequently, the characteristics of the inverter are not described in DE102014109513A1.

[0007] Furthermore, the subject of DE102016103883A1 is DC bus leakage detection for use in electric vehicles. By definition, DE102016103883A1 relates to electrical DC parameters, but this does not include the observation of AC connectors.

[0008] In addition, EP2256506A1 and EP3620800A1 disclose some schemes for monitoring the entire non-closed power grid, which can also only be applied to a limited extent to closed systems consisting of inverters and AC libraries. Summary of the Invention

[0009] Therefore, the objective of this invention is to provide a protection device for an inverter that can function normally during emergency backup grid operation.

[0010] According to the present invention, the task is solved by performing an insulation test during emergency backup power grid operation, wherein the AC fault current flowing between the AC connector and the ground potential is determined and the AC insulation resistance is calculated from the AC fault current.

[0011] Preferably, the AC insulation resistance can be calculated using the voltage corresponding to the AC insulation resistance of the inverter, and the AC insulation resistance can be calculated from the AC fault current and the voltage corresponding to the AC insulation resistance, as is well known from electrical engineering. Such a voltage can be the output AC voltage (AC voltage) applied to the output side between the AC terminals of the inverter, as described at the beginning. "Corresponding to the AC insulation resistance" should be understood here as the voltage mentioned enabling the calculation of the AC insulation resistance.

[0012] In addition to the AC voltage applied to the output side between the AC terminals of the inverter, the AC insulation resistance can also be calculated using a voltage associated with this AC voltage applied between the AC terminals of the inverter. For example, the AC voltage applied between the AC terminals of the inverter can be derived from this voltage. This could be, for example, the voltage drop between the AC terminals of the inverter and the ground potential. If the voltage used to calculate the AC insulation resistance drops through the AC insulation resistance, then in the simplest case, the AC insulation resistance can also be calculated from the AC fault current and the corresponding voltage using Ohm's law.

[0013] Furthermore, the task is addressed by a monitoring device comprising a fault current calculation unit designed to calculate the AC fault current flowing between the AC connector and ground potential. The monitoring device also includes a calculation unit designed to calculate the AC insulation resistance using the AC fault current. Preferably, the calculation unit can use a voltage, such as AC voltage, according to the above embodiment to calculate the AC insulation resistance from the AC fault current and AC voltage. This enables monitoring of the AC insulation resistance during the ongoing operation of the inverter. The AC fault current is used here, which occurs when the AC insulation resistance between the AC connector and ground potential is small, i.e., when an AC insulation fault exists. If no insulation fault exists in the inverter, there is a large AC insulation resistance between the AC connector and ground potential, therefore only a small and negligible AC fault current flows between the AC connector and ground potential. The AC insulation resistance can be considered to tend to infinity. However, if an insulation fault exists, then a small AC insulation resistance and a large AC fault current flow between the AC connector and ground potential. That is, the insulation fault, i.e., the decreasing AC insulation resistance, can be identified by the calculated AC fault current.

[0014] The monitoring unit can be an integrated component of the inverter or constructed separately. The inverter and AC power bank constitute an IT (Isolé Terre) system in emergency standby grid operation (also known as islanded operation). The IT system is ungrounded, therefore, unlike the TN grid, fault current protection switches and / or AC / DC measurement units are eliminated at the AC terminals to save costs. According to the invention, the AC insulation resistance between the AC terminals and the ground potential is actively monitored in emergency standby grid operation. Passive measurement principles are disadvantageous in comparison because they cannot identify symmetrical faults at the AC terminals or insulation faults at the DC terminals. If, for example, in single-phase emergency standby grid operation, a phase and neutral wire have the same insulation fault relative to the ground potential, then passive voltage measurements of that phase relative to the ground potential and neutral wire potential cannot confirm the fault according to existing technology because no displacement occurs.

[0015] According to the present invention, the AC fault current is calculated from the insulation voltage applied between an intermediate circuit junction and ground potential, which can be determined by a voltage measurement unit, and the insulation impedance located between the intermediate circuit junction and ground potential. The insulation impedance may be known beforehand. Alternatively, the AC fault current can be calculated from the insulation voltage applied between the intermediate circuit midpoint of the inverter and ground potential, and the insulation impedance located between the intermediate circuit midpoint and ground potential. Depending on the inverter's structure, the neutral wire may also be directly connected to the intermediate circuit midpoint of the inverter. The insulation voltage may include both DC and AC voltage components.

[0016] Therefore, the fault current calculation unit can be designed to calculate the AC fault current from the insulation voltage and insulation impedance, wherein the insulation voltage is applied between the intermediate circuit joint and the ground potential or between the intermediate circuit midpoint and the ground potential, and the insulation impedance is located between the intermediate circuit joint and the ground potential or between the intermediate circuit midpoint and the ground potential.

[0017] Insulation tests can be repeated multiple times, preferably periodically at a test clock rate, and particularly preferably periodically at a test clock rate within the range of seconds. Therefore, the test clock rate can be used to check for AC fault current flow and to infer insulation faults.

[0018] Preferably, a voltage measurement unit is provided, which is designed to measure the insulation voltage applied between the intermediate circuit joint and the ground potential or between the intermediate circuit midpoint and the ground potential. The fault current calculation unit is designed to calculate the AC fault current from the insulation voltage and the insulation impedance between the intermediate circuit joint and the ground potential or between the intermediate circuit midpoint and the ground potential.

[0019] The insulation impedance can be approximated by parasitic DC capacitance, such as the capacitance on the DC side of a PV generator, or by parasitic DC capacitance and parallel DC insulation resistance.

[0020] Advantageously, during insulation testing, preferably throughout the entire insulation test, a test signal with a test frequency greater than the AC voltage is applied to the AC voltage. The test signal advantageously has an amplitude of up to 120V. A fault current threshold of 10 mA and / or an AC insulation resistance threshold of 50 ohms / volt can be set. If no test signal is set, then the AC frequency of the AC voltage can be used for insulation testing.

[0021] Preferably, a signal, such as a signal in the form of an unresponsive optical display, is output within 10 seconds of the occurrence of an insulation fault.

[0022] The inverter can be switched to fault current operation and / or at least partially disconnected when the AC fault current exceeds the fault current threshold and / or when it falls below the insulation resistance threshold of the AC insulation resistance. The insulation resistance threshold can be selected such that the AC insulation resistance is within a standard range. In conventional IT systems, detecting an insulation fault does not trigger a disconnection; however, for inverters, immediate disconnection can be considered.

[0023] Preferably, DC insulation faults are detected by connecting a high-resistance resistor between one of the intermediate circuit terminals and the ground potential, and calculating the DC voltage component of the insulation voltage between the intermediate circuit terminal and the ground potential before and after connecting the high-resistance resistor. The ratio between the DC voltage components of the calculated insulation voltages provides information about the insulation resistance level. The higher the insulation resistance, the more pronounced the potential shift relative to the ground potential at the intermediate circuit terminal is caused by connecting the high-resistance resistor. Therefore, the insulation voltage can be calculated before and after closing the power switch, for example using a voltage measurement unit, and the DC insulation fault occurring at any location can be identified from the thus obtained measurement value of the insulation voltage. Preferably, this identification of DC insulation faults is performed not only before the inverter is turned on but also during ongoing emergency standby grid operation. The determination of DC insulation faults is particularly advantageous when there is no current separation between the AC and DC terminals of the inverter. Therefore, the overall insulation resistance consists of the AC insulation resistance and the DC insulation resistance.

[0024] The DC insulation resistance between the intermediate current terminal and the ground potential can also be measured using a DC fault current unit, which can be an AC / DC measuring unit (RCMU, Residual Current Monitoring Unit). The AC / DC measuring unit calculates the total current at the intermediate current terminal and thus operates on the principle of a fault current protection switch. The AC / DC measuring unit can essentially be used to identify DC insulation faults, but only when the DC fault current flows between the intermediate current terminal and the ground potential. However, if the DC insulation fault occurs at a location that would otherwise be at the ground potential in a fault-free environment, no DC fault current flows, and therefore the DC insulation fault cannot be identified when using an AC / DC measuring unit.

[0025] The inverter can be configured for grid operation, wherein, when switching from emergency standby grid operation to grid operation, the inverter's AC connector is disconnected from the AC energy bank and / or connected to the power grid to transfer energy from the DC energy source to the power grid. Interruption insulation testing is preferred during grid operation. This is particularly advantageous when protective measures such as fault current protection devices and overcurrent protection devices have already been implemented in the power grid. Attached Figure Description

[0026] Next, refer to Figure 1 The invention is explained in more detail in figures 5, which exemplarily, schematically, and non-limitingly illustrate advantageous designs of the invention. Wherein: Figure 1 An arrangement system including an inverter, an AC energy storage unit, a DC energy source, and a monitoring device is shown. Figure 2 An arrangement system with a fault current calculation unit as a fault current measurement unit is shown; Figure 3 An arrangement system with a voltage measuring unit for measuring the insulation voltage across the insulation impedance is shown; Figure 4 An alternative arrangement system for measuring insulation impedance is shown; Figure 5 shows the curves of AC frequency, test frequency, and related fault current, phase angle, AC voltage, fault current phase angle, and insulation resistance. Detailed Implementation

[0027] Figure 1 An arrangement system including an inverter 2, an AC energy storage 4, and a DC energy source 3 is shown. The AC terminals AC1 and AC2 of the inverter 2 are connected to the AC energy storage 4, and the DC terminals DC+ and DC- of the inverter 2 are connected to the DC energy source 3. The AC terminals AC1 and AC2 can, for example, constitute two phases of the inverter 2. The inverter 2 also includes an optional intermediate circuit with intermediate circuit terminals ZK+ and ZK-, between which a DC intermediate circuit voltage Uzk appears.

[0028] Alternatively, a basic multiphase design, such as a three-phase design, can be connected to a two-phase AC energy storage 4, wherein two of the three phases of the inverter 2 are connected to the AC energy storage 4. The multiphase, such as a three-phase AC energy storage 4 can also be connected to the AC terminals of the inverter 2, wherein multiple AC terminals, such as three AC terminals, are each composed of a phase of the inverter 2.

[0029] Inverter 2 converts the DC input voltage Ue provided by DC energy source 3 into AC voltage ua through DC intermediate circuit voltages ZK and ZK-, which is then supplied to AC energy reservoir 4. Therefore, energy is transferred from DC energy source 3 to AC energy reservoir 4. DC voltage Ue is applied between DC terminals DC+ and DC-, and AC voltage ua is applied between AC terminals AC1 and AC2. For example, a motor or other power-consuming device can be used as AC energy reservoir 4. For example, a battery / energy storage device and / or photovoltaic cells can be used as DC energy source 3.

[0030] Not only on the DC connectors DC+ and DC-, but also on the AC connectors AC1 and AC2, filter capacitors and / or filter inductors (not shown in the figure) can be installed respectively.

[0031] Inverter 2 can also be designed bidirectionally, meaning that the AC voltage ua applied to AC terminals AC1 and AC2 can be converted into DC intermediate circuit voltages ZK+ and ZK-, and further converted into DC input voltage Ue applied to DC terminals DC+ and DC-. Therefore, energy transfer from AC terminals AC1 and AC2 to the intermediate circuit is also possible, where, for example, a storage device / battery directly consumes PV current. The bidirectional inverter 2 is also called a hybrid inverter. Therefore, separate AC terminals can also be used to supply power to the power consumption devices, i.e., power can be supplied to the devices by a storage device / battery. The AC terminals can be opened using an AC splitter, thus isolating inverter 2 from the power grid.

[0032] In the diagram, inverter 2 is therefore in emergency standby grid operation NE. Because the AC disconnector is open, the power grid and AC disconnector are not shown for simplicity. If inverter 2 is switched to grid operation (not shown), then AC connectors AC1 and AC2 are connected to the power grid (closed AC disconnector), and AC energy storage 4 can be disconnected. If inverter 2 is switched back to emergency standby grid operation NE, then AC connectors AC1 and AC2 are disconnected from the power grid and connected to AC energy storage 4. Energy storage 4 can be, for example, at least one socket integrated into inverter 2, to which at least one power consumer can be connected and powered.

[0033] That is, in this variant, inverter 2 can power, for example, those consumers connected to the integrated socket during emergency standby grid operation (NE). All other consumers can be isolated from the AC splitter.

[0034] In another variant, the integrated socket may also be powered by the grid during grid operation. Here, the AC power bank, however, can still be technically separated from the circuitry and, to some extent, bridging to the connection to the power grid.

[0035] In emergency standby grid operation (NE), the grid protection device is ineffective on the power grid side because inverter 2 is disconnected from the power grid and therefore also disconnected from the grid protection device. More specifically, an IT system exists in conjunction with AC energy storage 4, which, by definition, is not connected to the ground potential GND. An AC insulation resistance R exists between the AC terminals AC1 and AC2 of inverter 2 and the ground potential GND. isoAC In this invention, the AC insulation resistance R used to determine the ohm is set. isoACThe monitoring device 1 includes a fault current calculation unit 10, which is designed to calculate the AC fault current i flowing between AC connectors AC1, AC2 and ground potential GND. fAC To perform insulation test I. Calculation unit 11 uses AC fault current i fAC Calculate AC insulation resistance R under the following conditions isoAC In AC insulation resistance R isoAC The "AC" in this context refers to the AC side of inverter 2, where the AC insulation resistance R appears. isoAC .

[0036] If no insulation fault occurs in inverter 2, then the AC insulation resistance R isoAC It is high. However, if an insulation fault occurs in inverter 2, then the AC insulation resistance R... isoAC The AC insulation resistance R decreases. That is, when an insulation fault occurs, the AC insulation resistance R... isoAC It is low.

[0037] If the fault current acquisition unit 10 is designed as a fault current measurement unit, such as Figure 2 As shown, the AC fault current i can be directly measured. fAC To perform insulation test I. An AC / DC measurement unit (RCMU, residual current monitoring unit) can be set up as fault current measurement unit 10, which surrounds AC connectors AC1 and AC2 and measures the total current.

[0038] The fault current acquisition unit 10 and the calculation unit 11 can be connected to the inverter’s control unit / regulation unit (not shown), which is designed to operate the fault current acquisition unit 10 and the calculation unit 11.

[0039] Alternatively or additionally, the fault current calculation unit 10 can be designed to calculate the AC fault current i fAC To perform insulation test I. For this purpose, the insulation voltage U can be determined between one of the intermediate circuit terminals ZK+ or ZK- and the ground potential GND. iso The AC voltage component, such as in Figure 3 As shown in the diagram. This is combined with the known insulation resistance Z between one of the intermediate circuit terminals ZK+ and ZK- and the ground potential GND. iso The AC fault current i can be calculated in the fault current calculation unit 10. fAC Then it can be obtained from the AC fault current i fAC Calculate the AC insulation resistance R accordingly. isoAC .like Figure 4As shown, especially in multiphase inverter 2, the insulation voltage U between the intermediate circuit midpoint ZKm and the ground potential GND can also be calculated. iso The AC voltage component. See above for reference. Figure 3 Calculate the AC fault current i as described fAC Insulation voltage U iso This can be obtained using voltage measurement unit 12, such as in Figure 3 and 4 As shown in the figure. The voltage measuring unit 12 preferably measures the insulation voltage U. iso The DC voltage component and the AC voltage component.

[0040] Preferably, during insulation test I, and especially throughout the entire insulation test, the test frequency f is greater than the AC frequency f of the AC voltage ua. AC The test signal s is modulated onto the AC voltage ua, which in Figures 1 to 4 The value is represented by "ua (+s)". To generate a test signal, a test signal generator can be set up. To modulate the test signal onto the AC voltage ua, a test signal coupling unit can be set up.

[0041] Preferably through the known parasitic DC capacitance C isoDC For example, the insulation impedance Z can be approximated by the DC side capacitor of a DC voltage source (such as a PV generator). iso DC capacitor C isoDC This may be necessary for other routine procedures in inverter 2 and is therefore preferably obtained continuously. In this case, in order to calculate the AC fault current i fAC The already calculated DC capacitor C can be retrieved. isoDC The insulation resistance Z can be considered as... iso It is almost equivalent to reactance Xc, which is obtained by passing through the AC voltage at the AC frequency f. AC The parasitic DC capacitance C at the test frequency f of the test signal s, or when using the test signal s. isoDC Formation. It can also be achieved through the known parasitic DC capacitance C. isoDC DC insulation resistance R of parallel ohms isoDC To approximate the insulation impedance Z iso , such as in Figure 3 As indicated in [the document]. In DC insulation resistance R isoDC and DC capacitor C isoDC The "DC" in this context refers to the DC side of inverter 2, where a DC insulation resistance R appears. isoDC and DC capacitor C isoDC Especially in terms of insulation resistance Z iso DC insulation resistance R of ohms isoDCWhen the value is small, the insulation resistance Z iso It can also be approximated using reactance Xc, because the DC insulation resistance R isoDC If the value is small or low, a DC insulation fault can be considered to exist, therefore the AC insulation resistance R can be omitted. isoAC The calculation.

[0042] In both cases, you can directly purchase from merchant i. fAC = U iso / Z iso (at AC frequency f) AC Calculate the AC fault current i at the test frequency f of the test signal s (or when using the test signal s). fAC The value and phase angle. To calculate the AC fault current i fAC Preferably at AC frequency f AC Or, when using test signal s at the test frequency f of test signal s, use the insulation voltage U. iso The AC voltage component.

[0043] The AC fault current is based on Figure 2 Measurement or based on Figure 1 , Figure 3 and / or Figure 4 The calculation is irrelevant to the AC fault current i fAC This is used as input to calculation unit 11, which calculates the AC insulation resistance R. isoAC Therefore, the following steps can be specified: In the next step, calculate the AC phase angle of the AC voltage ua (or the test signal s). With AC fault current i fAC Fault current phase angle Phase deviation between : = - That is, phase deviation. It is the AC voltage ua (or test signal s) and the AC fault current i fAC The zero-crossing distance between them. If the test signal s is superimposed on the AC voltage ua, then the component with the test frequency f of the test signal s is related to the obtained voltage ua+s.

[0044] As explained above, in DC insulation resistance R isoDC In high cases, it can be derived from known relationships. (Z corresponds to Z) iso Approximate Equation Formation Then the equation R can be solved. isoAC Therefore, the AC insulation resistance R can be calculated.isoAC .

[0045] It is preferable to calculate the AC insulation resistance R periodically, for example within a range of seconds. isoAC .

[0046] Similarly, the AC insulation resistance R can be calculated using a test signal s with a test frequency of f. isoAC With AC frequency f AC The superposition / modulation test frequency f can here be equal to the AC frequency f of the AC voltage ua. AC Or preferably at a higher frequency. The test signal s is preferably applied permanently during insulation test I.

[0047] In Figure 5, the AC frequency f will be used at 50Hz. AC The insulation test measurements (right) and (left) are compared using a test frequency f of 150 Hz.

[0048] The above figure shows a circuit with AC frequency f. AC and the insulation voltage U at the test frequency f iso The AC voltage component; in the figure below, you can see the AC frequency f. AC and AC fault current i at test frequency f fAC It can be seen that when using the test frequency f greater than the AC frequency f... AC (That is, the fault current i when the test signal s is 150Hz > 50Hz) fAC (or its amplitude) is greater than when using AC frequency f AC The fault current i corresponding to the time fAC higher.

[0049] Furthermore, the AC phase angle of the test signal s is shown on the left side of Figure 5. and the associated fault current phase angle (Dashed line), and the AC phase angle of the AC voltage is shown on the right side of Figure 5. and the associated fault current phase angle (Dashed line). This also shows the phase angle of the fault current at AC frequency f. Deviation from the fault current phase angle at the test frequency f Calculate the AC insulation resistance R during insulation test I. isoAC This is preferably repeated periodically at a test clock rate. The test clock rate can be in the range of seconds and, for example, 1 to 5 seconds, preferably 3 seconds, meaning that the insulation test I is repeated every 1 to 5 seconds, preferably every 3 seconds. If the AC phase angle of the AC voltage ua (or test signal s) is... Phase angle with fault current Phase deviation between Calculated AC insulation resistance R isoAC If the insulation resistance is lower than the predetermined insulation resistance threshold, then there is an AC insulation fault.

[0050] Because DC capacitor C isoDC Reactance X C As the frequency increases, the AC fault current i fAC Similarly, it increases with increasing frequency (see Figure 5, i) fAC Therefore, it can compensate for possible fluctuations in parasitic DC capacitance C. isoDC The effect of insulation voltage U. iso The measurement of the AC voltage component is unaffected or only slightly affected by the increase in frequency (this is shown here with the aid of the test frequency f = 150 Hz relative to the AC frequency f), because the reactance X is reduced despite the increase in current. C It also decreased to the same extent.

[0051] Because inverter 2 does not have current separation between the AC and DC terminals, DC insulation faults can also be detected additionally. This can be achieved using fault current determination unit 10 (e.g., by...). Figure 2 The AC / DC measurement unit (RCMU) is used to implement the fault current calculation unit to measure the fault current i. fAC It measures both the AC and DC voltage components. The AC / DC measurement unit is particularly suitable for detecting sudden DC insulation faults.

[0052] However, DC insulation faults can also typically be detected by connecting a high-resistance resistor R between one of the intermediate circuit terminals ZK+ and ZK- and the ground potential GND using a test switch S. Figure 3 As shown by the dashed line, and also for pressing Figure 4 The modified scheme is applicable. The test switch S can be controlled by the monitoring device 1.

[0053] DC insulation faults can be detected by connecting a high-resistance resistor R between one of the intermediate circuit terminals ZK+ and ZK- and the ground potential GND, and calculating the insulation voltage U between one of the intermediate circuit terminals ZK+ and ZK- and the ground potential GND before and after connecting the high-resistance resistor R. iso The DC voltage components. The ratio between these two DC voltage components can be used to deduce the insulation resistance R. isoDC The magnitude of the insulation resistance R. isoDC The higher the resistance, the more pronounced the potential shift relative to ground potential GND at the intermediate circuit terminals ZK+ and ZK- through the high-resistance resistor R. Insulation voltage U iso The larger the ratio between them, the greater the DC insulation resistance R. isoDC The larger it is.

[0054] In order to press Figure 2 In the variant schemes, to achieve sufficient speed and current resistance, it is advantageous to replace or supplement the switch set in the AC / DC fault current detector with a semiconductor switch to realize the test switch S. In all variant schemes, before and after connecting the high-resistance resistor R, the insulation voltage U between one of the intermediate circuit terminals ZK+, ZK- and the ground potential GND is calculated. iso The DC voltage component.

[0055] For combined insulation monitoring of DC and AC insulation faults, the following routine procedure is now preferred: 1. Check for DC insulation faults. 2. If a DC insulation fault is found, repeat the above check. 3. If there is no DC insulation fault, switch inverter 2 to grid operation or emergency standby grid operation. 4. Check for DC or AC insulation faults. This is performed periodically / serially at the test clock rate, following the procedure described earlier.

[0056] Step 4 may stipulate that, according to regulations, AC and DC insulation faults must be continuously checked in the IT network during the operation of the emergency backup power grid. When detecting AC or DC insulation faults, that is, at the AC insulation resistance and / or DC insulation resistance R... isoDC If the time is too short, inverter 2 can be shut off accordingly.

Claims

1. A method for determining the AC insulation resistance (R) between the AC terminals (AC1, AC2) of the inverter (2) and the ground potential (GND). isoAC The method, in which, A DC voltage (Ue) is applied between each DC connector (DC1, DC2) of the inverter (2) and an AC voltage (ua) is applied between each AC connector (AC1, AC2) of the inverter (2). The DC connectors (DC1, DC2) are connected to a DC energy source (3), and in emergency standby grid operation (NE), the AC connectors (AC1, AC2) are connected to an AC energy reservoir (4) to transfer energy from the DC energy source (3) to the AC energy reservoir (4). The characteristic feature is that an insulation test (I) is performed in emergency standby grid operation (NE), wherein an insulation voltage (Ue) is applied between one of the intermediate circuit connectors (ZK+, ZK-) and the ground potential (GND) or between the intermediate circuit midpoint (ZKm) and the ground potential (GND). iso And the insulation resistance (ZK+, ZK-) between one of the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND) or between the intermediate circuit midpoint (ZKm) and the ground potential (GND). iso Find the AC fault current (i) flowing between the AC terminals (AC1, AC2) and the ground potential (GND). fAC ), calculate the AC phase angle at AC voltage (ua) ( AC ) and AC fault current (i fAC The fault current phase angle () ifAC Phase deviation (Δ) between And from the phase deviation (Δ) Calculate AC insulation resistance (R) isoAC ).

2. The method according to claim 1, characterized in that, Repeated insulation tests (I).

3. The method according to claim 1 or 2, characterized in that, Through parasitic DC capacitance (C) isoDC ) or through parasitic DC capacitance (C isoDC ) and parallel DC insulation resistance (R) isoDC Approximate calculation of insulation impedance (Z) iso ).

4. The method according to claim 1 or 2, characterized in that, During the insulation test (I), the test frequency (f) is greater than the AC frequency (fa) of the AC voltage (ua). AC The test signal is modulated onto the AC voltage (ua).

5. The method according to claim 1 or 2, characterized in that, In AC fault current (I fAC When the fault current exceeds the fault current threshold and / or when it is below the AC insulation resistance (R isoAC When the insulation resistance threshold of ) is reached, the inverter (2) is switched to fault current operation and / or at least partially disconnected.

6. The method according to claim 1 or 2, characterized in that, The inverter (2) is set to grid operation (NB), wherein, when switching from emergency standby grid operation (NE) to grid operation (NB), the AC connectors (AC1, AC2) of the inverter (2) are disconnected from the AC energy bank (4) and / or connected to the power grid in order to transfer energy from the DC energy source (3) to the power grid, and the insulation test (I) is interrupted during grid operation (NB).

7. The method according to claim 1 or 2, characterized in that, DC insulation faults are detected by connecting a high-resistance resistor between the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND). The insulation voltage (U) between one of the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND) is calculated before and after connecting the high-resistance resistor. iso The DC voltage component of the sample is calculated, and the insulation voltage (U) of each sample is obtained. iso The ratio of DC insulation faults can be used to infer DC insulation faults.

8. The method according to claim 1 or 2, characterized in that, From the insulation voltage (U) iso AC voltage component and insulation resistance (Z) iso ) Calculate the AC fault current (i fAC ).

9. The method according to claim 2, characterized in that, The insulation test is repeated periodically at the test clock rate.

10. The method according to claim 9, characterized in that, The insulation test is repeated periodically at a test clock rate in the range of seconds.

11. Used to determine the AC insulation resistance (R) between the AC terminals (AC1, AC2) of the inverter (2) and the ground potential (GND). isoAC The monitoring device (1) of the ) wherein, The inverter (2)’s DC terminals (DC1, DC2) can be connected to a DC energy source (3), and the inverter (2)’s AC terminals (AC1, AC2) can be connected to an AC energy bank (4) during emergency standby grid operation (NE). The monitoring device (1) is characterized by including a fault current calculation unit (10), which is designed to calculate the fault current from the insulation voltage (U) applied between one of the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND) or between the intermediate circuit midpoint (ZKm) and the ground potential (GND). iso And the insulation resistance (ZK+, ZK-) between one of the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND) or between the intermediate circuit midpoint (ZKm) and the ground potential (GND). iso Find the AC fault current (i) flowing between the AC terminals (AC1, AC2) and the ground potential (GND). fAC ), and the monitoring device (1) includes a computing unit (11) designed to calculate the AC phase angle (uA) at AC voltage (uA). AC ) and AC fault current (i fAC The fault current phase angle () ifAC Phase deviation (Δ) between And from the phase deviation (Δ) Calculate AC insulation resistance (R) isoAC ).

12. The monitoring device (1) according to claim 11, characterized in that, A voltage measurement unit (12) is provided, which is designed to measure the insulation voltage (U) applied between one of the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND). iso ), and the fault current calculation unit (10) is designed to obtain the fault current from the insulation voltage (U iso The insulation resistance (Z) between the intermediate circuit terminals (ZK+, ZK-) and the ground potential (GND) iso ) Calculate AC fault current (i fAC ).

13. The monitoring device (1) according to claim 11, characterized in that, The fault current calculation unit is designed to obtain the fault current from the insulation voltage (U iso AC voltage component and insulation resistance (Z) iso ) Calculate the AC fault current (i fAC ).

14. The monitoring device (1) according to claim 12, characterized in that, The voltage measurement unit is designed to measure insulation voltage (U). iso The AC voltage component of the fault current is obtained from the insulation voltage (U). The fault current calculation unit (10) is designed to obtain the AC voltage component of the fault current from the insulation voltage (U). iso AC voltage component and insulation resistance (Z) iso ) Calculate AC fault current (i fAC ).

15. An arrangement system comprising an inverter (2) and a monitoring device (1) according to any one of claims 11 to 14.

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